Module Name: src
Committed By: riastradh
Date: Mon May 1 12:17:56 UTC 2023
Modified Files:
src/sys/kern: kern_mutex.c
Log Message:
mutex(9): Omit needless membar_consumer.
In practical terms, this is not necessary because MUTEX_SET_WAITERS
already issues MUTEX_MEMBAR_ENTER, which on all architectures is a
sequential consistency barrier, i.e., read/write-before-read/write,
subsuming membar_consumer.
In theoretical terms, MUTEX_MEMBAR_ENTER might imply only
write-before-read/write, so one might imagine that the
read-before-read ordering of membar_consumer _could_ be necessary.
However, the memory operations that are significant here are:
1. load owner := mtx->mtx_owner
2. store mtx->mtx_owner := owner | MUTEX_BIT_WAITERS
3. load owner->l_cpu->ci_curlwp to test if equal to owner
(1) is program-before (2) and at the same memory location,
mtx->mtx_owner, so (1) happens-before (2).
And (2) is separated in program order by MUTEX_MEMBAR_ENTER from (3),
so (2) happens-before (3).
So even if the membar_consumer were intended to guarantee that (1)
happens-before (3), it's not necessary, because we can already prove
it from MUTEX_MEMBAR_ENTER.
But actually, we don't really need (1) happens-before (3), exactly;
what we really need is (2) happens-before (3), since this is a little
manifestation of Dekker's algorithm between cpu_switchto and
mutex_exit, where each CPU sets one flag and must ensure it is
visible to the other CPUs before testing the other flag -- one flag
here is the MUTEX_BIT_WAITERS bit, and the other `flag' here is the
condition owner->l_cpu->ci_curlwp == owner; the corresponding logic,
in cpu_switchto, is:
1'. store owner->l_cpu->ci_curlwp := owner
2'. load mtx->mtx_owner to test if MUTEX_BIT_WAITERS set
To generate a diff of this commit:
cvs rdiff -u -r1.105 -r1.106 src/sys/kern/kern_mutex.c
Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.
Modified files:
Index: src/sys/kern/kern_mutex.c
diff -u src/sys/kern/kern_mutex.c:1.105 src/sys/kern/kern_mutex.c:1.106
--- src/sys/kern/kern_mutex.c:1.105 Wed Apr 12 06:35:40 2023
+++ src/sys/kern/kern_mutex.c Mon May 1 12:17:56 2023
@@ -1,4 +1,4 @@
-/* $NetBSD: kern_mutex.c,v 1.105 2023/04/12 06:35:40 riastradh Exp $ */
+/* $NetBSD: kern_mutex.c,v 1.106 2023/05/01 12:17:56 riastradh Exp $ */
/*-
* Copyright (c) 2002, 2006, 2007, 2008, 2019 The NetBSD Foundation, Inc.
@@ -40,7 +40,7 @@
#define __MUTEX_PRIVATE
#include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: kern_mutex.c,v 1.105 2023/04/12 06:35:40 riastradh Exp $");
+__KERNEL_RCSID(0, "$NetBSD: kern_mutex.c,v 1.106 2023/05/01 12:17:56 riastradh Exp $");
#include <sys/param.h>
#include <sys/atomic.h>
@@ -673,7 +673,6 @@ mutex_vector_enter(kmutex_t *mtx)
* If the waiters bit is not set it's unsafe to go asleep,
* as we might never be awoken.
*/
- membar_consumer();
if (mutex_oncpu(owner)) {
turnstile_exit(mtx);
owner = mtx->mtx_owner;
